High strength steel sheet and method for manufacturing the same

ABSTRACT

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2020/038196, filed Oct. 8, 2020 which claims priority to Japanese Patent Application No. 2019-192514, filed Oct. 23, 2019, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high strength steel sheet having excellent formability which is suitably used as a member in various industries, such as the automotive industry and the electrical industry, and a method for manufacturing the high strength steel sheet. Specifically, aspects of the present invention attempt to produce a high strength steel sheet that has a TS (tensile strength) of 980 MPa or more and is excellent in terms of not only ductility but also stretch flangeability and bendability.

BACKGROUND OF THE INVENTION

Improving fuel economy of automobiles has been an important issue from the viewpoint of global environmental conservation. Accordingly, there have been active movements to increase the strengths of car body materials in order to allow reductions in the thicknesses of the car body materials and the weights of car bodies. Since increasing the strength of a steel sheet may degrade the formability of the steel sheet, the development of a material having both high strength and high formability has been anticipated.

There has been proposed a high strength steel sheet that uses strain induced transformation of retained austenite, which has a high strength and excellent ductility. This steel sheet has a microstructure including retained austenite and can be readily formed into an intended shape due to retained austenite during forming, while the steel sheet comes to have a high strength subsequent to forming as a result of transformation of retained austenite to martensite.

For example, Patent Literature 1 proposes a high strength steel sheet having a tensile strength of 1000 MPa or more, a total elongation (EL) of 30% or more, and markedly high ductility, the steel sheet being manufactured using strain induced transformation of retained austenite. Such a steel sheet is manufactured by causing a steel sheet containing C, Si, and Mn as fundamental constituents to form austenite, subsequently quenching the steel sheet in the bainite transformation temperature range, and performing isothermal holding, that is, an austempering treatment. Retained austenite is formed as a result of C being concentrated at austenite due to the austempering treatment, and the formation of a large amount of retained austenite requires addition of a large amount of C exceeding 0.3%. However, an increase in the C concentration in steel degrades spot weldability, and in particular a C concentration exceeding 0.3% significantly degrades spot weldability. Thus, it has been difficult to bring this technique into active use for automotive steel sheets. In addition, no consideration is given to stretch flangeability and bendability in Patent Literature 1 because the main purpose is to improve the ductility of the high strength steel sheet.

In Patent Literature 2, a high Mn steel is used and a suitable strength-ductility balance is achieved by performing a heat treatment in the ferrite-austenite dual phase region. However, in Patent Literature 2, no study is performed on the improvement of ductility by concentrating Mn at untransformed austenite. Thus, there is a room for improving workability.

In Patent Literature 3, a medium Mn steel is used and total elongation is increased by performing a heat treatment in the ferrite-austenite dual phase region, concentrating Mn at untransformed austenite, and thereby forming stable retained austenite. However, no consideration is given to achievement of compatibility of elongation, stretch flangeability, and bendability. Moreover, no study is performed on the improvement of stretch flangeability and bendability by controlling the distribution of not only Mn but also C in the second phase composed of retained austenite and martensite. It is considered that the concentration of Mn is not at the level enough to achieve the compatibility of elongation, stretch flangeability, and bendability because the amount of time during which the heat treatment is performed in the manufacturing method described in Patent Literature 3 is small and the diffusion rate of Mn is low.

Furthermore, in Patent Literature 4, a medium Mn steel is used and uniform elongation and stretch flangeability are improved by performing a heat treatment of a hot rolled steel sheet in the ferrite-austenite dual phase region for a long period of time, facilitating the concentration of Mn at untransformed austenite, and thereby forming retained austenite grains having a high aspect ratio. However, in Patent Literature 4, the improvement of the ductility and stretch flangeability of a high strength steel sheet only by Mn concentration is studied; no study is performed on the compatibility of the improvement of stretch flangeability, bendability, and elongation by controlling the distribution of C and Mn in the second phase composed of retained austenite and martensite.

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. S61-157625

PTL 2: Japanese Unexamined Patent Application Publication No. H1-259120

PTL 3: Japanese Unexamined Patent Application Publication No. 2003-138345

PTL 4: Japanese Patent No. 6123966

SUMMARY OF THE INVENTION

Aspects of the present invention were made in light of the above-described circumstances. An object according to aspects of the present invention is to provide a high strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability and a method for manufacturing the high strength steel sheet. Note that the term “formability” used herein refers to ductility, stretch flangeability, and bendability.

In order to achieve the above object, the inventors of the present invention conducted extensive studies in terms of the chemical composition of the steel sheet and a method for manufacturing the steel sheet for manufacturing a high strength steel sheet having excellent formability and consequently found the following facts.

Specifically, the inventors of the present invention found that it is important to heat a steel slab including Mn at a content of 2.50% by mass or more and 8.00% by mass or less and optional other alloying elements, such as Ti, at adequate contents, hot roll the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then perform coiling at 300° C. or more and 750° C. or less, optionally perform holding at a temperature equal to or lower than the Ac₁ transformation temperature for more than 1800 s, then perform cold rolling, subsequently perform holding at a temperature equal to or higher than the Ac₃ transformation temperature−50° C. for 20 s or more and 1800 s or less, then perform cooling to a cooling stop temperature equal to or lower than the martensitic transformation start temperature, then perform reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently perform holding at the reheating temperature for 2 s or more and 600 s or less, and then perform cooling to room temperature such that film-like austenite grains at which C is concentrated, which serve as cores of fine retained austenite grains having a high aspect ratio in the subsequent annealing step, are formed. After the cooling has been performed, holding is performed to a temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, cooling is performed to 100° C. or less, holding is then performed at more than 100° C. and 400° C. or less for 10 s or more, and then cooling is performed. It was found that the above-described method enables the production of a high strength steel sheet having excellent formability, the steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more, and the product of the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite and the average aspect ratio of the retained austenite is 3.0 or more, wherein the quotient of the average C content (mass %) in the retained austenite divided by the average C content (mass %) in the ferrite is 3.0 or more, wherein the quotient of the average C content (mass %) in the retained austenite divided by the average Mn content (mass %) in the retained austenite is 0.05 or more, and wherein the content of diffusible hydrogen in steel is 0.3 ppm by mass or less.

Aspects of the present invention were made on the basis of the above-described findings. The summary of aspects of the present invention is as follows.

[1] A high strength steel sheet including a chemical composition containing, by mass, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: 2.50% or more and 8.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Al: 0.001% or more and 2.000% or less, with the balance being Fe and incidental impurities; a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein a quotient of an area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by a total area fraction of the retained austenite is 0.3 or more, wherein a quotient of an average Mn content (mass %) in the retained austenite divided by an average Mn content (mass %) in the ferrite is 1.5 or more, and a product of the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite and an average aspect ratio of the retained austenite is 3.0 or more, wherein a quotient of an average C content (mass %) in the retained austenite divided by an average C content (mass %) in the ferrite is 3.0 or more, wherein a quotient of the average C content (mass %) in the retained austenite divided by the average Mn content (mass %) in the retained austenite is 0.05 or more, and wherein a content of diffusible hydrogen in steel is 0.3 ppm by mass or less.

[2] The high strength steel sheet described in [1], wherein the chemical composition further contains at least one element selected from, by mass, Ti: 0.200%, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.0050% or less, and REM: 0.0050% or less.

[3] The high strength steel sheet described in [1] or [2], the high strength steel sheet further including a galvanized layer disposed on a surface of the high strength steel sheet.

[4] The high strength steel sheet described in [3], wherein the galvanized layer is a galvannealed layer.

[5] A method for manufacturing a high strength steel sheet, the method including heating a steel slab having the chemical composition described in [1] or [2], hot rolling the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then performing coiling at 300° C. or more and 750° C. or less, subsequently performing cold rolling, then performing holding at a temperature equal to or higher than an Ac₃ transformation temperature −50° C. for 20 s or more and 1800 s or less, subsequently performing cooling to a cooling stop temperature equal to or lower than a martensitic transformation start temperature, then performing reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently performing holding at the reheating temperature for 2 s or more and 600 s or less, then performing cooling to room temperature, subsequently performing holding at a temperature equal to or higher than an Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, then performing cooling to 100° C. or less, then performing holding at higher than 100° C. and 400° C. or less for 10 s or more, and subsequently performing cooling.

[6] The method for manufacturing a high strength steel sheet described in [5], the method further including, subsequent to the coiling and prior to the cold rolling, performing holding at a temperature equal to or lower than the Ac₁ transformation temperature for more than 1800 s.

[7] The method for manufacturing a high strength steel sheet described in [5] or [6], the method further including, subsequent to the holding at the temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.

[8] The method for manufacturing a high strength steel sheet described in [7], the method further including, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.

According to aspects of the present invention, a high strength steel sheet that has a TS (tensile strength) of 980 MPa or more and is excellent in terms of formability, that is, specifically, not only ductility but also stretch flangeability and bendability, can be produced. Applying a high strength steel sheet produced by the manufacturing method according to aspects of the present invention to, for example, automotive structural components reduces the weights of car bodies and thereby improves fuel economy. Thus, the use of the high strength steel sheet is highly valuable from an industrial viewpoint.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are specifically described below. Hereinafter, “%” used for describing the contents of elements means “% by mass” unless otherwise specified.

(1) The reasons for which the contents of the constituents of steel are limited to the above ranges in accordance with aspects of the present invention are described below.

C: 0.030% or More and 0.250% or Less

C is an element necessary for forming tempered martensite and thereby increasing strength. C is also an element effective for enhancing the stability of retained austenite and ductility of steel. If the C content is less than 0.030%, it becomes difficult to achieve the intended area fraction of tempered martensite and the intended strength may fail to be achieved. Moreover, it becomes difficult to achieve a sufficient area fraction of retained austenite and suitable ductility may fail to be achieved. If C is added to steel in an excessive amount such that the C content exceeds 0.250%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Furthermore, the weld zone and the heat-affected zone may become hardened significantly and, consequently, the mechanical properties of the weld zone may become degraded. Thus, spot weldability, arc weldability, and the like may become degraded. From the above-described viewpoints, the C content is limited to 0.030% or more and 0.250% or less. The C content is preferably 0.080% or more and 0.200% or less.

Si: 0.01% or More and 3.00% or Less

Si is effective for achieving suitable ductility since it enhances the strain hardenability of ferrite. If the Si content is less than 0.01%, the advantageous effects of the addition of Si may be poor. Thus, the lower limit is set to 0.01%. Adding Si to steel in an excessive amount such that the Si content exceeds 3.00% may cause steel embrittlement, which degrades ductility and bendability, and formation of red scale, which degrades surface quality. Furthermore, the quality of coating may become degraded. Accordingly, the Si content is limited to 0.01% or more and 3.00% or less. The Si content is preferably 0.20% or more and 2.00% or less and is more preferably 0.20% or more and less than 0.70%.

Mn: 2.50% or More and 8.00% or Less

Mn is an additional element extremely important in accordance with aspects of the present invention. Mn is an element that stabilizes retained austenite and is effective for achieving suitable ductility. Mn is also an element that increases steel strength by solid solution strengthening. Mn is also effective for forming stable retained austenite at which Mn is concentrated, thereby enabling the intended quotient of the average Mn content (mass %) in retained austenite divided by the average Mn content (mass %) in ferrite to be achieved, and achieving suitable ductility. The above-described actions are confirmed when the Mn content in steel is 2.50% or more. However, if Mn is added to steel in an excessive amount such that the Mn content exceeds 8.00%, a nonuniform microstructure may be formed due to the segregation of Mn and, consequently, stretch flangeability may become degraded. Furthermore, the intended quotient of the average C content (mass %) in retained austenite divided by the average Mn content (mass %) in retained austenite may fail to be achieved. This makes it difficult to achieve both suitable ductility and bendability. Furthermore, ease of conversion treatment and the quality of coating may become degraded. From the above-described viewpoints, the Mn content is limited to 2.50% or more and 8.00% or less. The Mn content is preferably 3.10% or more and 6.00% or less and is more preferably 3.20% or more and 4.20% or less.

P: 0.001% or More and 0.100% or Less

P is an element that causes solid solution strengthening and may be added to steel in accordance with the intended strength. P is also an element that facilitates ferrite transformation and is therefore effective for forming a multiphase microstructure. In order to produce the above-described advantageous effects, it is necessary to limit the P content to 0.001% or more. If the P content exceeds 0.100%, weldability may become degraded. Moreover, in the case where the alloying treatment of zinc coating is performed, the alloying rate may be reduced and the quality of the zinc coating may become degraded. Accordingly, the P content is limited to 0.001% or more and 0.100% or less and is preferably 0.005% or more and 0.050% or less.

S: 0.0001% or More and 0.0200% or Less

S segregates at grain boundaries to cause steel embrittlement during hot working and is present in the form of sulfides, which degrade local deformability. Accordingly, it is necessary to limit the S content to 0.0200% or less. The S content is preferably 0.0100% or less and is more preferably 0.0050% or less. However, due to the limitations of production technology, it is necessary to limit the S content to 0.0001% or more. Accordingly, the S content is limited to 0.0001% or more and 0.0200% or less. The S content is preferably 0.0001% or more and 0.0100% or less and is more preferably 0.0001% or more and 0.0050% or less.

N: 0.0005% or More and 0.0100% or Less

N is an element that degrades the aging resistance of steel. The degradation of aging resistance becomes particularly significant if the N content exceeds 0.0100%. Although it is preferable to minimize the N content, it is necessary to limit the N content to 0.0005% or more due to the limitations of production technology. Accordingly, the N content is limited to 0.0005% or more and 0.0100%. The N content is preferably 0.0010% or more and 0.0070% or less.

Al: 0.001% or More and 2.000% or Less

Al is an element that widens the ferrite-austenite dual phase region and is effective for reducing the dependence of mechanical properties on the annealing temperature, that is, enhancing the stability of mechanical properties. The lower limit is set to 0.001% because the advantageous effects of addition of Al are poor if the Al content is less than 0.001%. Al is also an element that serves as a deoxidizing agent and is effective for the index of cleanliness of steel. Thus, it is preferable to add Al to steel in the deoxidization step. However, adding Al to steel in an excessive amount such that the Al content exceeds 2.000% increases the risk of steel slabs cracking during continuous casting and reduces manufacturability. From the above viewpoints, the Al content is limited to 0.001% or more and 2.000% or less. The Al content is preferably 0.200% or more and 1.200% or less.

In addition to the above-described constituents, the chemical composition may contain at least one element selected from, by mass, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.0050% or less, and REM (abbreviation for rare earth metals): 0.0050% or less.

Ti: 0.200% or Less

Ti is effective for the precipitation strengthening of steel. Ti increases the strength of ferrite, thereby reduces the difference in hardness between ferrite and the hard second phase (tempered martensite or retained austenite), and consequently enables further suitable stretch flangeability to be achieved. In the case where Ti is added to steel, the Ti content is preferably 0.005% or more and is further preferably 0.010% or more. However, if the Ti content exceeds 0.200%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Accordingly, in the case where Ti is added to steel, the Ti content is limited to 0.200% or less. The Ti content is preferably 0.100% or less.

Nb: 0.200% or Less, V: 0.500% or Less, and W: 0.500% or Less

Nb, V, and W are effective for the precipitation strengthening of steel. Furthermore, similarly to the advantageous effects of addition of Ti, Nb, V, and W increase the strength of ferrite, thereby reduce the difference in hardness between ferrite and the hard second phase (tempered martensite or retained austenite), and consequently enable further suitable stretch flangeability to be achieved. In the case where Nb, V, and W are added to steel, the Nb, V, and W contents are each preferably 0.005% or more and are each more preferably 0.010% or more. However, if the Nb content exceeds 0.200% or the V or W content exceeds 0.500%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Accordingly, in the case where Nb is added to steel, the Nb content is limited to 0.200% or less. The Nb content is preferably 0.100% or less. In the case where V or W is added to steel, the V or W content is limited to 0.500% or less. The V or W content is preferably 0.300% or less.

B: 0.0050% or Less

B inhibits the formation and growth of ferrite at austenite grain boundaries. B increases the strength of ferrite, thereby reduces the difference in hardness between ferrite and the hard second phase (tempered martensite or retained austenite), and consequently enables further suitable stretch flangeability to be achieved. In the case where B is added to steel, the B content is preferably 0.0003% or more. However, if the B content exceeds 0.0050%, formability may become degraded. Accordingly, in the case where B is added to steel, the B content is limited to 0.0050% or less. The B content is preferably 0.0030% or less.

Ni: 1.000% or Less

Ni is an element that stabilizes retained austenite and is effective for achieving further suitable ductility. Ni is also an element that increases steel strength by solid solution strengthening. In the case where Ni is added to steel, the Ni content is preferably 0.005% or more. However, if the Ni content exceeds 1.000%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Accordingly, in the case where Ni is added to steel, the Ni content is limited to 1.000% or less.

Cr: 1.000% or Less and Mo: 1.000% or Less

Cr and Mo may be added to steel as needed because they enhance the balance between strength and ductility. In the case where Cr and Mo are added to steel, the Cr and Mo contents are preferably Cr: 0.005% or more and Mo: 0.005% or more. However, if Cr and Mo are added to steel in excessive amounts such that the Cr content exceeds 1.000% and the Mo content exceeds 1.000%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Accordingly, in the case where the above elements are added to steel, the contents of the elements are limited to Cr: 1.000% or less and Mo: 1.000% or less.

Cu: 1.000% or Less

Cu may be added to steel as needed because it is an element effective for strengthening of steel. If Cu is added to steel, the Cu content is preferably 0.005% or more. However, if Cu is added to steel in an excessive amount such that the Cu content exceeds 1.000%, the area fraction of hard tempered martensite is increased to an excessive level. In such a case, during a hole expanding test, the amount of microvoids formed at the grain boundaries of tempered martensite may be increased and the propagation of cracks may proceed disadvantageously. Thus, stretch flangeability may become degraded. Accordingly, in the case where Cu is added to steel, the Cu content is limited to 1.000% or less.

Sn: 0.200% or Less and Sb: 0.200% or Less

Sn and Sb are added to steel as needed in order to inhibit a region of the surface layer of the steel sheet which has a thickness of several tens of micrometers from being decarburized as a result of nitriding or oxidation of the surface of the steel sheet. Sn and Sb are effective for inhibiting the above nitriding or oxidation, preventing a reduction in the area fraction of tempered martensite in the surface of the steel sheet, and achieving certain strength and the stability of mechanical properties. In the case where Sn and Sb are added to steel, the Sn and Sb contents are each preferably 0.002% or more. However, if any of the above elements is added to steel in an excessive amount such that the content of the element exceeds 0.200%, toughness may become degraded. Accordingly, in the case where Sn and Sb are added to steel, the Sn and Sb contents are each limited to 0.200% or less.

Ta: 0.100% or Less

Similarly to Ti and Nb, Ta increases strength by forming an alloy carbide and an alloy carbonitride. Furthermore, it is considered that Ta partially dissolves in Nb carbide or carbonitride to form a composite precipitate, such as (Nb,Ta) (C,N), thereby markedly reduces the coarsening of the precipitates, and stabilizes the increase in strength by precipitation strengthening. Therefore, Ta is preferably added to steel. In the case where Ta is added to steel, the Ta content is preferably 0.001% or more. Even if Ta is added to steel in an excessive amount, the above precipitation stabilization effect may become saturated and the costs of the alloying elements are also increased. Accordingly, in the case where Ta is added to steel, the Ta content is limited to 0.100% or less.

Ca: 0.0050% or Less, Mg: 0.0050% or Less, Zr: 0.0050% or Less, and REM: 0.0050% or Less

Ca, Mg, Zr, and REMs are elements effective for increasing the sphericity of sulfides and reducing the adverse effects of the sulfides on stretch flangeability. In the case where the above elements are added to steel, the contents of the elements are each preferably 0.0005% or more. However, if any of the above elements is added to steel in an excessive amount such that the content of the element exceeds 0.0050%, the amount of inclusions and the like may be increased and, consequently, surface and internal defects may occur. Accordingly, if Ca, Mg, Zr, and REMs are added to steel, the Ca, Mg, Zr, and REM contents are each limited to 0.0050% or less.

The constituent other than the above-described constituents, that is, the balance, includes Fe and incidental impurities.

(2) The steel microstructure is described below.

Area Fraction of Ferrite: 30% or More and 80% or Less

It is necessary to limit the area fraction of ferrite to 30% or more in order to achieve sufficient ductility. It is also necessary to limit the area fraction of soft ferrite to 80% or less in order to achieve a tensile strength of 980 MPa or more. Note that the term “ferrite” used herein refers to polygonal ferrite, granular ferrite, and acicular ferrite, that is, ferrite materials that are relatively soft and rich in ductility. The area fraction of ferrite is preferably 40% or more and 75% or less.

Area Fraction of Tempered Martensite: 3.0% or More and 35% or Less

The area fraction of tempered martensite needs to be 3.0% or more in order to achieve a high local elongation, suitable stretch flangeability, suitable bendability, and a high yield ratio. It is necessary to limit the area fraction of tempered martensite to 3.0% or more in order to achieve a high local elongation, suitable stretch flangeability, and suitable bendability. It is also necessary to limit the area fraction of tempered martensite to 35% or less because ductility and bendability may become degraded if the fraction of tempered martensite is excessively high. The area fraction of tempered martensite is preferably 5.0% or more and 20% or less.

The area fractions of ferrite and tempered martensite can be determined by grinding a thickness-wise cross section (L-section) of the steel sheet which is parallel to the rolling direction, etching the L-section in 3-vol% nital, then observing the ¼-thickness position (i.e., the position ¼ of the thickness of the steel sheet below the surface in the depth direction) of the L-section with a SEM (scanning electron microscope) at a 2000-fold magnification for 10 fields of view, calculating the area fractions of the microstructure components (ferrite and tempered martensite) with “Image-Pro” produced by Media Cybernetics, Inc. for each of the 10 fields of view on the basis of the resulting microstructure images, and taking the averages thereof. In the above microstructure images, ferrite appears as a gray microstructure component (base microstructure) and tempered martensite appears as a microstructure component that includes a white (As-quenched) martensite portion and a gray inner structure formed inside the martensite portion.

Area Fraction of Retained Austenite: 8% or More

It is necessary to limit the area fraction of retained austenite to 8% or more in order to achieve sufficient ductility. The area fraction of retained austenite is preferably 12% or more and 25% or less.

The area fraction of retained austenite was determined by grinding the steel sheet to a position 0.1 mm below the ¼-thickness position, further grinding the steel sheet 0.1 mm by chemical polishing, then measuring the integrated intensity ratios of diffraction peaks on the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron with an X-ray diffraction apparatus using CoKα radiation, and taking the averages of the nine integrated intensity ratios.

Quotient of Area Fraction of Retained Austenite Grains Having Aspect Ratio of 2.0 or More and Minor Axis Length of 1 μm or Less Divided by Total Area Fraction of Retained Austenite Is 0.3 or More

Limiting the quotient of the area fraction of retained austenite grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less divided by the total area fraction of the retained austenite to 0.3 or more is an important condition constituting an aspect of the present invention. The retained austenite grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less reduce the likelihood of voids being formed during punching performed prior to the flange forming step and thereby enhance stretch flangeability. For achieving suitable stretch flangeability, the area fraction of retained austenite grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less needs to be high, in addition to satisfying the area fraction of retained austenite which is high enough to achieve high ductility. The above index is preferably 0.5 or more. The upper limit for the above aspect ratio is preferably 15.0 or less. The lower limit for the above minor axis length is preferably 0.05 μm or more, which is the detection limit in EBSD.

Martensite and retained austenite were identified using a phase map obtained by EBSD (electron backscattered diffraction). The aspect ratio of a retained austenite grain was calculated by drawing an ellipse circumscribing the retained austenite grain with “Photoshop elements 13” and dividing the length of the major axis of the ellipse by the length of the minor axis of the ellipse.

Quotient of Average Mn Content (Mass %) in Retained Austenite Divided by Average Mn Content (Mass %) in Ferrite: 1.5 or More

Limiting the quotient of the average Mn content (mass %) in retained austenite divided by the average Mn content (mass %) in ferrite to 1.5 or more is an extremely important condition constituting an aspect of the present invention. For achieving suitable ductility, the area fraction of stable retained austenite at which Mn is concentrated needs to be high. The above index is preferably 2.0 or more. Although the upper limit for the above quotient is not limited because the higher the average Mn content in retained austenite, the higher the ductility, the above index is preferably 10.0 or less because the increase in ductility becomes saturated if the above index exceeds 10.0.

Product of Quotient of Average Mn Content (Mass %) in Retained Austenite Divided by Average Mn Content (Mass %) in Ferrite and Average Aspect Ratio of Retained Austenite Grains is 3.0 or More

Limiting the product of the quotient of the average Mn content (mass %) in retained austenite divided by the average Mn content (mass %) in ferrite and the average aspect ratio of retained austenite grains to 3.0 or more is an extremely important condition. For achieving suitable ductility, the area fraction of stable retained austenite grains having a high aspect ratio at which Mn is concentrated needs to be high. The above index is preferably 4.0 or more. The upper limit for the above index is suitably 20.0 or less.

Quotient of C Content (Mass %) in Retained Austenite Divided by Average C Content (Mass %) in Ferrite: 3.0 or More

Limiting the quotient of the C content (mass %) in retained austenite divided by the average C content (mass %) in ferrite to 3.0 or more is an extremely important condition constituting an aspect of the present invention. When the C content in retained austenite is increased, retained austenite remains even in high strain region during bending deformation and, consequently, bendability is enhanced. Therefore, for achieving suitable ductility and bendability, the area fraction of stable retained austenite at which Mn is concentrated needs to be high. The above index is preferably 5.0 or more. The upper limit for the above index is preferably 10.0 or less.

Quotient of Average C Content (Mass %) in Retained Austenite Divided by Average Mn Content (Mass %) in Retained Austenite is 0.05 or More

Limiting the quotient of the C content (mass %) in retained austenite divided by the Mn content (mass %) in retained austenite to 0.05 or more is an extremely important condition constituting an aspect of the present invention. For achieving suitable bendability, the area fraction of stable retained austenite at which not only Mn but also C are concentrated needs to be high. When not only Mn but also C are concentrated at retained austenite, stable retained austenite can be present in high strain region and, consequently, compatibility of suitable ductility and bendability can be achieved. The above index is preferably 0.06 or more. The upper limit for the above index is preferably 0.10 or less.

The C and Mn contents in retained austenite and ferrite are determined with 3-dimensional atom probe (3DAP) using a sample taken from the ¼-thickness position. First, a portion of the steel sheet which includes retained austenite and ferrite is sampled and then formed into an acicular sample using focused ion beam. A voltage is applied to the acicular sample with the 3DAP, and C and Mn ions released upon the voltage application are analyzed. The Mn content can be determined in terms of atom% by dividing the numbers of C and Mn atoms measured by the total number of all the atoms measured, for each of retained austenite and ferrite. The above measurement is performed for each of 30 retained austenite grains and 30 ferrite grains which are randomly selected in the field of view, and the averages of the C and Mn contents determined by the quantitative analysis are calculated. The C and Mn contents (mass %) in retained austenite and ferrite can be obtained by converting the C and Mn contents (atom%) to a mass basis.

Content of Diffusible Hydrogen in Steel is 0.3 ppm by Mass or Less

Limiting the content of diffusible hydrogen in steel to 0.3 ppm by mass or less is an important condition constituting an aspect of the present invention. For achieving a high local elongation and suitable stretch flangeability, it is necessary to limit the content of diffusible hydrogen in steel to 0.3 ppm by mass or less. The content of diffusible hydrogen in steel is preferably 0.2 ppm by mass or less. A test specimen having a length of 30 mm and a width of 5 mm was taken from an annealed steel sheet. After the coated layer had been removed by grinding, the content of diffusible hydrogen in steel and the peak of release of diffusible hydrogen were measured. The release peak was measured by thermal desorption spectrometry (TDS) at a heating rate of 200° C./hr. The hydrogen detected at 300° C. or less was considered diffusible hydrogen.

The advantageous effects according to aspects of the present invention are not impaired even if the steel microstructure according to aspects of the present invention contains quenched martensite, bainite, pearlite, or carbides, such as cementite, in addition to ferrite, tempered martensite, and retained austenite, when the area fraction of the quenched martensite, bainite, pearlite, or carbides is 10% or less.

The steel sheet may include a galvanized layer disposed on the surface. The galvanized layer may be a galvannealed layer formed by performing an alloying treatment of the galvanized layer.

(3) Manufacturing conditions are described below.

Temperature at which Steel Slab is Heated

The temperature at which the slab is heated is preferably, but not limited to, 1100° C. or more and 1300° C. or less. Since the precipitates present when the steel slab is heated are present in the form of coarse precipitates in the final steel sheet and do not affect strength, the Ti- and Nb-based precipitates formed during casting can be again dissolved. It is also preferable to limit the temperature at which the steel slab is heated to 1100° C. or more in order to scale off air bubbles, segregation, and the like present in the slab surface layer, further reduce cracks and irregularities present in the surface of the steel sheet, and thereby further flatten the surface of the steel sheet. The temperature at which the steel slab is heated is preferably 1300° C. or less in order to reduce scale loss caused with an increase in oxidation. The above slab heating temperature is more preferably 1150° C. or more and 1250° C. or less.

The steel slab is preferably manufactured by a continuous casting process in order to prevent macrosegregation. Alternatively, the steel slab may be manufactured by ingot casting, thin-slab casting, or the like. In addition to the conventional method in which, subsequent to the production of the steel slab, the steel slab is cooled to room temperature and then reheated, energy-saving processes, such as hot charge rolling and hot direct rolling, in which the steel slab is not cooled to room temperature but charged into a heating furnace while the temperature of the steel slab is high or the steel slab is rolled immediately after heat insulation has been done for a short period of time, may be used without problems. The slab is formed into a sheet bar by rough rolling under common conditions. In the case where the heating temperature is relatively low, the sheet bar is preferably heated with a bar heater or the like prior to finish rolling in order to prevent the occurrence of troubles during hot rolling.

Finish Rolling Delivery Temperature in Hot Rolling: 750° C. or More and 1000° C. or Less

The heated steel slab is hot rolled to form a hot rolled steel sheet by rough rolling and finish rolling. If the above finish rolling delivery temperature exceeds 1000° C., the amount of oxides (scales) formed rapidly increases, the roughness of the interface between the base iron and the oxides is increased accordingly, and surface quality may become degraded subsequent to pickling and cold rolling. Furthermore, if hot rolling scales partially remain unremoved subsequent to pickling, they adversely affect ductility and stretch flangeability. Moreover, grain size may be increased to an excessive level and the roughness of the surface of a pressed article may be increased during working. If the above finish rolling delivery temperature is less than 750° C., the rolling force is increased, the rolling load is increased accordingly, and the rolling reduction ratio performed while austenite is in a non-recrystallization state is increased. In such a case, an abnormal texture develops. Consequently, the in-plane anisotropy of the final product is increased significantly. This degrades the uniformity of material quality (stability of mechanical properties). In addition, the aspect ratio of retained austenite grains is reduced and, consequently, ductility and stretch flangeability may become degraded. Accordingly, it is necessary to limit the finish rolling delivery temperature in hot rolling to 750° C. or more and 1000° C. or less. The finish rolling delivery temperature is preferably 800° C. or more and 950° C. or less.

Temperature at which Coiling is Performed Subsequent to Hot Rolling: 300° C. or More and 750° C. or Less

If the temperature at which coiling is performed subsequent to hot rolling exceeds 750° C., the grain size of ferrite included in the microstructure of the hot rolled steel sheet is increased, the aspect ratio of retained austenite grains included in the final annealed steel sheet is reduced, it becomes difficult to achieve the intended quotient of the area fraction of retained austenite grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less divided by the total area fraction of the retained austenite, and stretch flangeability becomes degraded consequently. If the temperature at which coiling is performed subsequent to hot rolling is less than 300° C., the strength of the hot rolled steel sheet is increased. In such a case, the rolling load required for cold rolling may be increased and defects of the shape of the steel sheet may occur. This reduces productivity. Accordingly, it is necessary to limit the temperature at which coiling is performed subsequent to hot rolling to 300° C. or more and 750° C. or less. The above coiling temperature is preferably 400° C. or more and 650° C. or less.

The finish rolling may be performed in a continuous manner by joining the rough rolled steel sheets to one another during hot rolling. The rough rolled steel sheets may be coiled temporarily. In order to reduce the rolling force required for hot rolling, a part or the entirety of the finish rolling may be performed using a lubricant. It is also preferable to perform lubrication rolling in order to increase the uniformity in the shape of the steel sheet and the uniformity of material quality. In the case where lubrication rolling is performed, the coefficient of friction at which the lubrication rolling is performed is preferably 0.10 or more and 0.25 or less.

The hot rolled steel sheet produced in the above-described manner may be pickled optionally. It is preferable to perform pickling because it enables the removal of oxides present on the surface of the steel sheet and further improves ease of conversion treatment and the quality of coating. The pickling may be performed in a single stage or multiple stages.

Holding at Temperature Equal to or Lower than Ac₁ Transformation Temperature for More than 1800 s

It is preferable to perform holding at a temperature equal to or lower than the Ac₁ transformation temperature for more than 1800 s because it softens the steel sheet that is to be cold rolled in the subsequent step. If holding is performed at a temperature higher than the Ac₁ transformation temperature, Mn concentrates at austenite, hard martensite and retained austenite are formed subsequent to cooling, and the steel sheet may fail to be softened. In such a case, in the subsequent annealing step, retained austenite may be formed at the grain boundaries and the amount of retained austenite grains having a low aspect ratio may be increased. This makes it difficult to achieve the intended quotient of the area fraction of retained austenite grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less divided by the total area fraction of the retained austenite and may consequently degrade stretch flangeability. Even at a temperature equal to or lower than the Ac₁ transformation temperature, if the holding time is 1800 s or less, it becomes difficult to eliminate the strain that remains subsequent to hot rolling and the steel sheet may fail to be softened.

The heat treatment may be performed by any annealing method, such as continuous annealing or batch annealing. Subsequent to the above heat treatment, cooling is performed to room temperature. The method with which the cooling is performed and the rate at which the cooling is performed are not limited. Any cooling method, such as furnace cooling or natural cooling in batch annealing, or gas jet cooling, mist cooling, or water cooling in continuous annealing, may be used. In the case where a pickling treatment is performed, a conventional method may be used.

Cold Rolling

The resulting steel sheet is cold rolled. The cold rolling reduction ratio is preferably, but not limited to, 15% or more and 80% or less. Performing cold rolling at the above reduction ratio enables formation of an sufficiently recrystallized, intended microstructure and further improves ductility.

Holding at Temperature Equal to or Higher Than Ac₃ Transformation Temperature−50° C. for 20 s or More and 1800 s or Less

If holding is performed at a temperature less than the Ac₃ transformation temperature−50° C., Mn concentrates at austenite, martensitic transformation does not occur during cooling, and cores of retained austenite grains having a high aspect ratio may fail to be formed consequently. In such a case, in the subsequent annealing step, retained austenite may be formed at the grain boundaries disadvantageously. This increases the amount of retained austenite grains having a low aspect ratio and makes it impossible to form the intended microstructure. Even at a temperature equal to or higher than the Ac₃ transformation temperature−50° C., if the holding time is less than 20 s, recrystallization does not occur to a sufficient degree and the intended microstructure fails to be formed. Consequently, ductility may become degraded. If the above holding is performed for more than 1800 s, the amount of hard tempered martensite is increased subsequent to the final annealing treatment and ductility and bendability may become degraded consequently.

Cooling to Cooling Stop Temperature Equal to or Lower than Martensitic Transformation Start Temperature

If the cooling stop temperature is higher than the martensitic transformation start temperature, when the amount of martensite that is to be transformed is small, all of the untransformed austenite may transform to martensite during the final cooling, and cores of retained austenite grains having a high aspect ratio fail to be formed. In such a case, in the subsequent annealing step, retained austenite may be formed at the grain boundaries disadvantageously. This increases the amount of retained austenite grains having a low aspect ratio and makes it impossible to form the intended microstructure. The cooling stop temperature is preferably equal to or higher than the martensitic transformation start temperature−250° C. and equal to or lower than the martensitic transformation start temperature−50° C.

Reheating to Reheating Temperature of 120° C. or More and 450° C. or Less and Holding at Reheating Temperature for 2 s or More and 600 s or Less

If the reheating temperature is less than 120° C., C is not concentrated at retained austenite formed in the subsequent annealing step and the intended microstructure fails to be formed consequently. If the reheating temperature exceeds 450° C., the decomposition of cores of retained austenite grains having a high aspect ratio occurs and the amount of retained austenite grains having a low aspect ratio is increased accordingly. This makes it impossible to form the intended microstructure. If the holding time is less than 2 s, similarly, the cores of retained austenite grains having a high aspect ratio cannot be formed and the intended microstructure fails to be formed. If the holding time exceeds 600 s, the decomposition of cores of retained austenite grains having a high aspect ratio occurs and the amount of retained austenite grains having a low aspect ratio is increased accordingly. This makes it impossible to form the intended microstructure. Cooling is performed to room temperature subsequent to the reheating and holding. The cooling method is not limited. For example, it is preferable to perform natural cooling, gas cooling, or mist cooling to 200° C. or less and then perform water cooling. Pickling may be performed as needed. In the case where pickling is performed, common picking methods may be used.

Holding at Temperature Equal to or Higher than Ac₁ Transformation Temperature and Equal to or Lower than Ac₁ Transformation Temperature+150° C. for 20 s or More and 600 s or Less

Performing holding at a temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less is an extremely important condition constituting an aspect of the present invention. If the holding is performed at a temperature lower than the Ac₁ transformation temperature for less than 20 s, the carbides formed during heating remain undissolved, it becomes difficult to form sufficient area fractions of tempered martensite and retained austenite, and strength is reduced consequently. If the above holding temperature exceeds the Ac₁ transformation temperature+150° C., the area fraction of martensite is increased subsequent to the annealing treatment, the amount of tempered martensite present subsequent to the final annealing treatment is increased to an excessive level, and the intended microstructure fails to be formed consequently. Furthermore, Mn and C fail to concentrate at austenite at sufficient levels and, consequently, ductility and bendability become degraded. In addition, a sufficient area fraction of retained austenite fails to be formed and ductility becomes degraded consequently. The above holding temperature is preferably equal to or lower than the Ac₁ transformation temperature+100° C. If the holding time exceeds 600 s, the area fraction of martensite is increased subsequent to the annealing treatment, the amount of tempered martensite present subsequent to the final annealing treatment is increased to an excessive level, and the intended microstructure fails to be formed consequently. Furthermore, Mn and C fail to concentrate at austenite at sufficient levels and, consequently, ductility and bendability become degraded. In addition, the area fraction of retained austenite which is enough to achieve the intended ductility fails to be formed.

Cooling to 100° C. or Less

Performing cooling to 100° C. or less enables the amount of martensite which is large enough to achieve the certain tensile strength to be formed. The cooling stop temperature is preferably about 20° C. or more and about 50° C. or less in consideration of the manufacturing costs.

Galvanizing Treatment

In the case where a galvanizing treatment is performed, the galvanizing treatment is performed before the holding is performed at the temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less and then the cooling is performed to 100° C. or less. Since this cooling treatment is performed in order to increase coatability by controlling the temperature of the steel sheet that is to be dipped into the plating bath, the cooling is performed to a temperature equal to or higher than the temperature of the galvanizing bath and equal to or lower than about 600° C. or less. The cooling method is not limited. For example, natural cooling, furnace cooling, and gas jet cooling may be used. In the case where a hot-dip galvanizing treatment is performed, the steel sheet that has been subjected to the above annealing treatment is dipped into a galvanizing bath having a temperature of 440° C. or more and 500° C. or less in order to perform a hot-dip galvanizing treatment. Then, the weight of coating is adjusted by gas wiping or the like. For hot-dip galvanizing, a galvanizing bath having an Al content of 0.08% or more and 0.30% or less is preferably used. Instead of a hot-dip galvanizing treatment, an electrogalvanizing treatment or the like may be used.

In the case where an alloying treatment of zinc coating is performed, subsequent to the galvanizing treatment, an alloying treatment of zinc coating is performed at 450° C. or more and 600° C. or less. If the alloying treatment is performed at more than 600° C., untransformed austenite may transform into pearlite, and the intended area fraction of retained austenite may fail to be achieved. Consequently, ductility may become degraded. Accordingly, in the case where an alloying treatment of zinc coating is performed, it is preferable to perform an alloying treatment of zinc coating at 450° C. or more and 600° C. or less.

Although the other conditions for the manufacturing method are not limited, it is preferable to perform the above annealing treatment using a continuous annealing facility from the viewpoint of productivity. It is also preferable to perform the set of treatments, such as annealing, hot-dip galvanizing, and alloying of zinc coating, using CGL (continuous galvanizing line), which is a hot-dip galvanizing line.

Holding at More than 100° C. and 400° C. or Less for 10 s or More

Performing holding at more than 100° C. and 400° C. or less for 10 s or more as a final heat treatment (final annealing treatment) is important in accordance with aspects of the present invention. If holding is performed at 100° C. or less or for less than 10 s, a sufficient area fraction of tempered martensite fails to be formed. Furthermore, diffusible hydrogen included in the steel is not released from the steel sheet. This results in degradation of stretch flangeability and bendability. If holding is performed at more than 400° C., a sufficient area fraction of retained austenite fails to be formed due to the decomposition of retained austenite. Consequently, steel ductility becomes degraded.

The above “high strength steel sheet” and the “high strength galvanized steel sheet” that includes the high strength steel sheet and a galvanized layer disposed on the surface may be subjected to skin pass rolling in order to, for example, perform shape correction and the adjustment of surface roughness. The rolling reduction ratio in the skin pass rolling is preferably 0.1% or more and 2.0% or less. If the above rolling reduction ratio is less than 0.1%, the advantageous effects are small and it is difficult to control the reduction ratio. Thus, this is considered the lower limit for the suitable rolling reduction ratio. If the rolling reduction ratio exceeds 2.0%, productivity may become degraded significantly. Thus, this is considered the upper limit for the suitable rolling reduction ratio. The skin pass rolling may be performed either online or offline. Skin pass rolling at an intended rolling reduction ratio may be performed in a single stage or multiple stages. Furthermore, various coating treatments, such as resin coating and fat coating, may be used.

EXAMPLES

Molten steels having the chemical compositions described in Table 1 with the balance being Fe and incidental impurities were prepared with a converter and formed into slabs by continuous casting process. The slabs were reheated to 1250° C. and then formed into high strength cold rolled steel sheets (CR) having a thickness of 1.0 mm or more and 1.8 mm or less under the conditions described in Tables 2 and 3. Furthermore, a galvanizing treatment was performed to produce hot-dip galvanized steel sheets (GI). An alloying treatment of the hot-dip galvanized steel sheets was performed to produce hot-dip galvannealed steel sheets (GA). The hot-dip galvanizing bath used for producing the hot-dip galvanized steel sheets (GI) was a zinc bath containing Al: 0.19% by mass. The hot-dip galvanizing bath used for producing the hot-dip galvannealed steel sheets (GA) was a zinc bath containing Al: 0.14% by mass. The bath temperature was 465° C. The coating weight was 45 g/m² per side (both sides were coated). In the production of the GA steel sheets, the Fe concentration in the coated layer was adjusted to 9% by mass or more and 12% by mass or less. The cross-sectional steel microstructure, tensile properties, and stretch flangeability of each of the above steel sheets were determined. Tables 4, 5, and 6 list the results.

TABLE 1 Steel Chemical composition (mass %) type C Si Mn P S N Al Ti Nb V W B Ni Cr Mo Cu Sn A 0.166 0.54 3.54 0.022 0.0021 0.0032 0.029 0.050 — — — — — — — — — B 0.182 0.74 3.24 0.028 0.0027 0.0041 0.045 0.042 — — — — — — — — — C 0.179 1.80 3.60 0.017 0.0019 0.0023 0.032 0.032 — — — — — — — — — D 0.239 0.96 3.30 0.028 0.0028 0.0025 0.056 — — — — — — — — — — E 0.049 1.00 4.12 0.031 0.0026 0.0024 0.033 — — — — — — — — — — F 0.181 2.90 3.99 0.027 0.0018 0.0027 0.029 0.044 — — — — — — — — — G 0.198 0.61 3.50 0.032 0.0020 0.0037 0.039 0.050 — — — — — — — — — H 0.081 1.01 5.12 0.024 0.0028 0.0029 0.046 — — — — — — — — — — I 0.178 1.47 3.77 0.022 0.0022 0.0023 0.036 — — — — — — — — — — J 0.160 0.20 3.52 0.027 0.0025 0.0037 0.031 0.043 — — — — — — — — — K 0.127 0.33 5.94 0.026 0.0026 0.0031 0.028 0.052 — — — — — — — — — L 0.193 0.45 3.11 0.024 0.0024 0.0030 0.033 — — — — — — — — — — M 0.154 0.63 4.18 0.024 0.0026 0.0032 0.038 — — — — — — — — — — N 0.171 0.51 3.22 0.030 0.0020 0.0042 0.038 0.040 — — — — — — — — — O 0.158 0.81 3.45 0.013 0.0019 0.0036 0.712 0.039 — — — — — — — — — P 0.154 0.57 3.57 0.021 0.0029 0.0028 1.179 0.041 — — — — — — — — — Q 0.201 0.32 3.53 0.026 0.0029 0.0041 0.225 — — — — — — — — — — R 0.022 0.45 3.55 0.020 0.0021 0.0034 0.031 0.045 — — — — — — — — — S 0.206 4.08 3.48 0.028 0.0024 0.0034 0.032 — — — — — — — — — — T 0.184 0.32 8.24 0.026 0.0024 0.0028 0.036 — — — — — — — — — — U 0.158 0.72 2.21 0.020 0.0020 0.0028 0.030 0.041 — — — — — — — — — V 0.168 0.60 2.56 0.018 0.0021 0.0039 0.042 0.263 — — — — — — — — — W 0.145 0.75 3.25 0.020 0.0024 0.0039 0.039 — 0.062 — — — — — — — — X 0.159 0.71 4.50 0.028 0.0026 0.0042 0.043 0.009 0.042 — — — — — — — — Y 0.121 1.13 3.58 0.032 0.0023 0.0029 0.043 0.090 — 0.059 — — — — — — — Z 0.102 1.17 4.09 0.029 0.0025 0.0033 0.043 — — — 0.022 — — — — — — AA 0.149 0.34 3.44 0.029 0.0024 0.0046 0.038 0.021 — — — 0.0015 — — — — — AB 0.191 0.70 5.93 0.025 0.0021 0.0040 0.012 0.015 — — — — 0.047 — — — — AC 0.093 0.51 6.37 0.020 0.0026 0.0039 0.058 0.061 — — — — — 0.082 — — — AD 0.147 0.72 3.58 0.022 0.0020 0.0034 0.041 0.050 — — — — — 0.501 — — — AE 0.098 1.42 3.07 0.027 0.0028 0.0034 0.033 0.025 — — — — — — 0.06 — — AF 0.105 0.52 3.58 0.023 0.0024 0.0025 0.039 — — — — — — — — 0.157 — AG 0.120 0.56 3.15 0.026 0.0020 0.0032 0.036 0.034 — — — — — — — — 0.006 AH 0.162 0.44 3.26 0.018 0.0020 0.0027 0.032 0.090 — — — — — — — — — AI 0.134 0.68 3.60 0.017 0.0017 0.0029 0.028 — — — — — — — — — — AJ 0.202 0.42 3.00 0.032 0.0030 0.0025 0.031 — 0.040 — — — — — — — 0.005 AK 0.214 0.22 3.71 0.025 0.0027 0.0038 0.030 — 0.031 — — — — — — — — AL 0.209 0.94 3.98 0.026 0.0023 0.0042 0.041 — — — — — — — — — — AM 0.196 1.23 3.80 0.021 0.0025 0.0034 0.035 — — — — — — — — — — AN 0.242 0.02 3.03 0.027 0.0021 0.0027 0.040 0.007 — — — — — — — — — AO 0.078 0.07 6.14 0.021 0.0028 0.0035 0.043 — — — — — — — — — — Ac₁ Ac₃ transfor- transfor- Ms mation mation temper- temper- temper- Steel Chemical composition (mass %) ature ature ature type Sb Ta Ca Mg Zr REM (° C.) (° C.) (° C.) Remark A — — — — — — 351 655 771 Invention steel B — — — — — — 358 666 785 Invention steel C — — — — — — 344 667 816 Invention steel D — — — — — — 336 665 766 Invention steel E — — — — — — 369 646 793 Invention steel F — — — — — — 328 668 858 Invention steel G — — — — — — 342 657 770 Invention steel H — — — — — — 318 617 753 Invention steel I — — — — — — 338 659 785 Invention steel J — — — — — — 354 652 756 Invention steel K — — — — — — 269 586 701 Invention steel L — — — — — — 359 666 754 Invention steel M — — — — — — 330 638 741 Invention steel N — — — — — — 362 664 776 Invention steel O — — — — — — 378 661 920 Invention steel P — — — — — — 389 655 1001 Invention steel Q — — — — — — 345 652 772 Invention steel R — — — — — — 401 656 818 Comparative steel S — — — — — — 340 695 903 Comparative steel T — — — — — — 157 521 597 Comparative steel U — — — — — — 407 695 818 Comparative steel V — — — — — — 390 683 890 Comparative steel W — — — — — — 370 666 777 Invention steel X — — — — — — 316 630 738 Invention steel Y — — — — — — 364 661 834 Invention steel Z — — — — — — 352 648 784 Invention steel AA — — — — — — 361 656 760 Invention steel AB — — — — — — 246 589 683 Invention steel AC — — — — — — 263 578 717 Invention steel AD — — — — — — 347 663 791 Invention steel AE — — — — — — 393 679 837 Invention steel AF — — — — — — 370 654 765 Invention steel AG — — — — — — 383 667 791 Invention steel AH 0.075 — — — — — 364 662 793 Invention steel AI — 0.087 — — — — 360 656 764 Invention steel AJ — — — — — — 360 668 754 Invention steel AK — 0.009 — — — — 328 646 721 Invention steel AL — — 0.0034 — — — 319 647 748 Invention steel AM — — — 0.0029 — — 330 655 768 Invention steel AN — — — — 0.0035 — 345 663 731 Invention steel AO — — — — — 0.0028 278 579 681 Invention steel Underlined part: Outside of the scope of the present invention, —: Content at the level of incidental impurities

The martensitic transformation start temperature, that is, the Ms temperature, and the Ac₁ and Ac₃ transformation temperatures were calculated using the following formulae.

Martensitic transformation start temperature: Ms temperature (° C.)=550−350×(% C)−40×(% Mn)−10×(% Cu)−17×(% Ni)−20×(% Cr)−10×(% Mo)−35×(% V)−5×(% W)+30×(% Al)

Ac₁ Transformation temperature (° C.)=751−16×(% C)+11×(% Si)−28×(% Mn)−5.5×(% Cu)−16×(% Ni)+13×(% Cr)+3.4×(% Mo)

Ac₃ Transformation temperature (° C.)=910−203≤(% C)+45×(% Si)−30×(% Mn)−20×(% Cu)−15×(% Ni)+11×(% Cr)+32×(% Mo)+104×(% V)+400×(% Ti)+200×(% Al)

where (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr), (% Mo), (% V), (% Ti), (% V), (% W), and (% Al) each represent the content (mass %) of the element.

TABLE 2 Heat treatment of hot rolled Annealing treatment of cold steel sheet rolled steel sheet Finish Heat Heat Reheating rolling treat- Heat Cold treat- Heat Cooling temper- delivery Coiling ment treat- rolling ment treat- stop Reheating ature temper- temper- temper- ment reduction temper- ment temper- temper- holding Steel ature ature ature time ratio ature time ature ature time No. type (° C.) (° C.) (° C.) (s) (%) (° C.) (s) (° C.) (° C.) (s) 1 A 890 530 550 21600 66.7 800 135 130 270 180 2 A 870 520 525 19800 58.8 850 160 170 300 350 3 A 860 500 590 32400 50.0 820 140 150 275 120 4 A 890 470 540 23400 64.7 815 120 120 300 250 5 A 900 530 510 18000 62.5 600 150 200 350 170 6 A 930 510 610 21600 58.8 910  15 215 280 140 7 A 860 540 615 14400 50.0 780 2400   80 150 220 8 A 800 550 570 32400 52.9 740 200 410 440  80 9 A 820 540 56.3 790 220 125 480 215 10 A 860 440 600 18000 64.7 810 120  70 110 320 11 A 900 530 510  4400 61.1 795  50 120 190 1000  12 A 885 370 535 12600 66.7 820 360 200 270  1 13 A 940 490 46.2 845 250 180 275 400 14 B 900 490 570 21600 54.8 830 800 315 420 540 15 C 900 520 550 18000 52.9 840 150 250 300 180 16 A 690 530 560 14400 47.1 880 180 140 215 210 17 A 850 840 590 32400 47.8 860 240 120 240 120 18 A 870 610 740 25200 58.8 870 180 110 200  50 19 A 860 550 430  7200 58.8 790 240 125 195 370 20 A 860 540 530 14400 68.4 780 300 175 330 530 21 A 900 550 530 10800 72.2 770 150 150 170 180 22 A 860 580 540 19800 64.7 740 180 130 250 260 23 A 890 585 560  3600 61.1 800  90 150 225 120 24 A 840 590 56.3 820 120 210 310 150 25 A 925 540 64.7 815 180 200 300  50 26 D 895 560 570 28800 58.8 880 360 140 250 180 27 E 950 550 570 18000 58.8 800 1200  180 310 240 28 F 865 600 530 14400 42.9 820 150 175 295 180 29 G 880 620 490 27000 57.1 830 140 100 250 130 30 H 885 500 535 36000 53.3 830 120 150 230 270 31 I 900 550 530 23400 50.0 765 150 140 340 540 32 J 880 500 510 25200 64.7 770 180 130 300  30 33 K 870 410 510 32400 48.6 790  90  65 200 220 Annealing treatment of cold Final annealing rolled steel sheet treatment Heat Heat treat- Heat treat- Heat ment treat- Alloying ment treat- temper- ment temper- temper- ment ature time ature ature time No. (° C.) (s) (° C.) (° C.) (s) Type* 1 695 140 120 24400 CR 2 700 170 180  5400 GI 3 690 160 520 150 32400 GA 4 710 310 530 200  3600 GA 5 670 130 180 64800 GI 6 700 130 550 310 10800 GA 7 710 125 500 320 14400 GA 8 700 150 220 60000 GI 9 730  90 460 130 24400 GA 10 660 530 480 330  7200 GA 11 690 450 210  9000 GI 12 700 180 140 21600 CR 13 690 120 545 170 25200 GA 14 700 160 200 28800 GI 15 710 150 520 220  600 GA 16 730 240 210 14400 GI 17 665 180 550 150 27000 GA 18 710 150 370   60 GI 19 510 300 510 300 23400 GA 20 880 480 485 160 25200 GA 21 685  5 525 310 18000 GA 22 715 2200  530 390 14400 GA 23 700 130 490  30 57600 GA 24 720 140 550 520 18000 GA 25 680 100 500 250   5 GA 26 705 110 370  2400 CR 27 690 480 355 10800 GI 28 770  65 255 36000 CR 29 710 260 530 260 12000 GA 30 690  90 510 320 12000 GA 31 710 300 525 350 72000 GA 32 690 210 300  4800 CR 33 660 120 120 96000 GI Underlined part: Outside of the scope of the present invention *CR: Cold rolled steel sheet (without coating), GI: Hot-dip galvanized steel sheet (without alloying of zinc coating), GA: Hot-dip galvannealed steel sheet

TABLE 3 Heat treatment of hot rolled Annealing treatment of cold steel sheet rolled steel sheet Finish Heat Heat Reheating rolling treat- Heat Cold treat- Heat Cooling temper- delivery Coiling ment treat- rolling ment treat- stop Reheating ature temper- temper- temper- ment reduction temper- ment temper- temper- holding Steel ature ature ature time ratio ature time ature ature time No. type (° C.) (° C.) (° C.) (s) (%) (° C.) (s) (° C.) (° C.) (s) 34 L 960 570 560 19800 46.2 790 100 180 240 150 35 M 890 620 580 39600 62.5 710 150 130 280 150 36 N 880 600 520 28800 62.5 790 120 130 290 180 37 O 880 530 520 7200 58.8 920 130 250 355 570 38 P 860 490 52.0 980 140 150 200 400 39 Q 860 560 590 9000 56.3 990 85 210 325 80 40 R 890 525 58.8 910 150 200 350 100 41 S 890 640 540 10800 62.5 880 600 130 200 200 42 S 870 610 550 10800 62.5 890 720 250 300 180 43 T 860 480 480 10800 64.7 800 60 50 180 190 44 U 895 550 550 36000 57.1 890 110 220 300 140 45 V 860 600 550 32400 50.0 860 330 200 400 240 46 W 900 500 56.3 900 125 220 320 100 47 X 910 560 510 36000 46.2 850 150 180 320 225 48 Y 870 550 580 10800 52.9 840 140 55 330 170 49 Z 895 330 47.1 815 300 180 300 510 50 AA 880 600 540 32400 55.6 830 900 140 405 260 51 AB 900 530 540 14400 56.3 890 140 150 180 240 52 AC 880 740 520 19800 58.8 790 60 120 250 160 53 AD 910 600 520 21600 53.3 890 220 150 300 90 54 AE 880 500 530 27000 64.7 810 120 140 325 210 55 AF 900 510 560 9000 62.5 840 180 210 320 540 56 AG 910 580 520 28800 56.3 830 150 200 320 210 57 AH 845 570 61.5 810 160 150 200 180 58 AI 900 560 530 32400 56.3 900 120 90 180 80 59 AJ 910 560 540 14400 56.3 910 80 100 200 125 60 AK 860 550 540 14400 56.3 800 90 180 210 150 61 AL 870 520 510 7200 64.7 825 150 170 370 150 62 AM 830 500 46.7 840 180 100 300 240 63 AN 850 500 560 21600 50.0 830 140 95 300 60 64 AO 890 510 515 12600 57.1 825 240 120 380 320 Annealing treatment of cold Final annealing rolled steel sheet treatment Heat Heat treat- Heat treat- Heat ment treat- Alloying ment treat- temper- ment temper- temper- ment ature time ature ature time No. (° C.) (s) (° C.) (° C.) (s) Type* 34 720 580 170 188000 GI 35 695 180 540 380 240 GA 36 705 120 290 24000 CR 37 770 240 560 230 9000 GA 38 790 360 510 240 9600 GA 39 780 140 270 14400 CR 40 680 180 530 200 10800 GA 41 720 110 480 270 72000 GA 42 700 100 340 1200 CR 43 660 300 120 60000 GI 44 725 150 210 5400 GI 45 710 180 150 54000 CR 46 695 300 530 200 96000 GA 47 730 180 520 260 4200 GA 48 695 600 160 12000 CR 49 690 240 510 290 72000 GA 50 700 90 540 200 4800 GA 51 720 180 270 96000 GI 52 710 250 120 188000 GI 53 720 180 525 120 24000 GA 54 700 150 190 10800 GI 55 710 60 550 210 24000 GA 56 730 45 500 150 28800 GA 57 740 250 510 230 18000 GA 58 725 180 310 2400 GI 59 690 150 520 190 27000 GA 60 715 240 180 36000 CR 61 700 120 480 255 188000 GA 62 700 90 260 25200 CR 63 685 210 550 370 30 GA 64 690 220 540 140 9600 GA Underlined part: Outside of the scope of the present invention *CR: Cold rolled steel sheet (without coating), GI: Hot-dip galvanized steel sheet (without alloying of zinc coating), GA: Hot-dip galvannealed steel sheet

The tensile test was conducted in accordance with JIS Z 2241 (2011) using a JIS No. 5 test specimen prepared by taking a sample from each of the steel sheets such that the tensile direction was perpendicular to the rolling direction of the steel sheet. In the tensile test, TS (tensile strength) and EL (total elongation) were measured. As for mechanical properties, an evaluation of “Good” was given in the following cases.

When TS was 980 MPa or more and less than 1080 MPa, EL≥20%

When TS was 1080 MPa or more and less than 1180 MPa, EL≥16%

Stretch flangeability was measured in accordance with JIS Z 2256 (2010). Specifically, each of the steel sheets was cut into a piece having a size of 100 mm×100 mm. A hole having a diameter of 10 mm was formed in the piece at a clearance of 12%±1%. Subsequently, while the piece was held by a die having an inside diameter of 75 mm at a blank holding force of 9 ton, a conical punch having a 60 degree apex angle was inserted into the hole. Then, the diameter of the hole was measured at the critical point at which cracking occurred. A critical hole expanding ratio λ(%) was calculated using the following formula. The evaluation of stretch flangeability was made on the basis of the critical hole expanding ratio.

Critical hole expanding ratio λ(%)={(D _(f) −D ₀)/D ₀}×100

where D_(f) represents the diameter (mm) of the hole at which cracking occurred, and D_(o) represents the initial diameter (mm) of the hole. In accordance with aspects of the present invention, an evaluation of “Good” was given in the following cases depending on TS.

When TS was 980 MPa or more and less than 1080 MPa, λ≥20%

When TS was 1080 MPa or more and less than 1180 MPa, λ≥15%

The bend test was conducted in accordance with the V block method described in JIS Z 2248 (1996) using a bend test specimen having a width of 30 mm and a length of 100 mm which was taken from each of the annealed steel sheets such that the rolling direction was parallel to the bending axis (bending direction). The test was conducted at a stroke speed of 100 mm/sec and n=3 for each bend radius. The presence of cracks in the outside portion of the bend part was determined with a stereoscopic microscope. The minimum bend radius at which cracking did not occur was considered the critical bend radius R. In accordance with aspects of the present invention, an evaluation of “Good” was given in terms of the bendability of the steel sheet in the case where the critical bend radius at 90° V bending R/t≤2.5 (t: thickness of the steel sheet) was satisfied.

The high strength steel sheets prepared in Invention examples all had a TS of 980 MPa or more. In Invention examples, high strength steel sheets having excellent formability were prepared. In contrast, in Comparative examples, at least one of the properties TS, EL, λ, and bendability was poor.

TABLE 4 Area fraction of RA having Average aspect ratio of Mn content 2.0 or more Average in RA/ and minor axis Mn content Average Area Area Area length of 1 μm Average Average in RA/ Average Mn content fraction fraction fraction or less/Total Mn content Mn content Average aspect in F × Average Steel Thickness of F of TM of RA area fraction in RA in F Mn content ratio aspect ratio No. type (mm) (%) (%) (%) of RA (mass %) (mass %) in F of RA of RA 1 A 1.2 55.6 18.9 20.6 0.341 6.57 2.12 3.10 5.41 16.77 2 A 1.4 53.5 20.0 19.0 0.390 6.80 2.85 2.39 4.35 10.38 3 A 1.6 54.4 20.4 22.2 0.492 6.47 2.78 2.33 4.89 11.38 4 A 1.2 48.8 24.4 17.6 0.546 6.30 2.73 2.31 5.12 11.82 5 A 1.2 49.8 18.0 22.3 0.175 6.55 2.18 3.00 4.44 13.34 6 A 1.4 28.8 15.4 21.8 0.413 5.29 3.14 1.68 3.99  6.72 7 A 1.4 45.8 37.3 16.3 0.304 6.77 2.80 2.42 4.55 11.00 8 A 1.6 45.9 30.3 15.2 0.195 6.12 2.45 2.50 1.03  2.57 9 A 1.4 51.2 25.3 15.9 0.215 5.82 2.91 2.00 1.22  2.44 10 A 1.2 54.2 19.2 20.2 0.497 8.77 2.61 3.36 3.69 12.39 11 A 1.4 55.3 15.4 19.5 0.181 6.32 2.87 2.20 1.24  2.73 12 A 1.2 55.9 15.5 19.6 0.137 6.10 2.82 2.16 1.35  2.92 13 A 1.4 54.9 18.9 20.3 0.592 7.23 1.72 4.20 5.14 21.61 14 B 1.4 50.8 20.3 22.1 0.613 5.05 1.23 4.11 4.21 17.28 15 C 1.6 51.6 18.5 24.9 0.444 6.10 1.82 3.35 3.55 11.90 16 A 1.8 55.7 23.7 18.1 0.224 5.61 2.46 2.28 1.12  2.55 17 A 1.2 60.6 20.0 15.5 0.213 6.01 0.86 6.97 2.64 18.39 18 A 1.4 48.3 27.2 19.9 0.031 6.47 2.97 2.18 1.93  4.20 19 A 1.4 78.4  2.4  7.2 0.338 4.54 2.55 1.78 2.52  4.49 20 A 1.2 53.0 37.4  5.6 0.382 3.86 2.99 1.29 3.86  4.98 21 A 1.0 75.6  2.3  6.5 0.347 4.92 3.05 1.61 4.70  7.58 22 A 1.2 56.4 35.8  5.5 0.338 3.57 2.79 1.28 5.13  6.56 23 A 1.4 51.9  2.5 18.2 0.422 5.99 2.87 2.09 4.32  9.02 24 A 1.4 56.7 30.5  7.1 0.501 6.01 2.76 2.18 4.89 10.65 25 A 1.2 45.3  2.1 17.7 0.409 6.11 2.80 2.18 5.62 12.26 26 D 1.4 60.5 18.9 18.4 0.630 5.38 2.71 1.99 5.38 10.68 27 E 1.4 53.2 14.6 23.4 0.484 4.54 2.78 1.64 6.20 10.14 28 F 1.6 54.7 22.2 19.2 0.418 4.16 2.11 1.97 5.14 10.13 29 G 1.2 46.4 29.0 15.5 0.360 6.78 2.45 2.77 3.88 10.74 30 H 1.4 55.4 20.6 12.5 0.497 8.03 3.10 2.59 3.98 10.31 31 I 1.4 53.2 24.5 15.3 0.701 5.87 2.89 2.03 4.57  9.28 32 J 1.2 53.3 18.8 25.8 0.360 4.66 2.25 2.07 5.38 11.15 33 K 1.8 49.4 22.0 24.3 0.741 7.00 4.45 1.57 4.67  7.35 34 L 1.4 58.7 20.4 15.2 0.304 6.26 2.88 2.17 6.33 13.76 35 M 1.2 51.1 21.2 23.3 0.574 8.26 2.47 3.34 5.47 18.27 36 N 1.2 53.6 19.4 17.1 0.401 5.98 2.91 2.05 8.30 17.06 37 O 1.4 52.9 19.9 19.7 0.331 7.79 3.21 2.43 6.44 15.63 38 P 1.2 50.2 18.9 25.8 0.411 7.89 1.91 4.14 3.30 13.65 39 Q 1.4 48.5 28.2 14.2 0.352 4.84 1.07 4.52 4.98 22.51 40 R 1.4 79.8  2.5  7.4 0.316 6.01 2.44 2.46 2.89  7.12 41 S 1.4 51.5 14.8 25.0 0.667 7.16 2.67 2.68 5.12 13.73 42 S 1.2 53.1 14.4 25.9 0.646 6.86 2.97 2.31 4.46 10.30 43 T 1.2 49.7 15.3 26.3 0.306 9.95 6.12 1.63 6.49 10.55 44 U 1.2 58.7 28.7  6.4 0.410 3.75 2.64 1.42 4.22  5.99 45 V 1.6 44.5 37.5 16.1 0.472 4.93 2.12 2.32 5.43 12.61 46 W 1.4 55.4 16.9 26.7 0.585 6.32 3.10 2.04 4.66  9.50 47 X 1.4 56.6 19.1 19.5 0.301 4.72 2.99 1.58 5.32  8.40 48 Y 1.6 52.7 19.8 24.5 0.305 5.24 2.41 2.17 4.29  9.33 49 Z 1.8 58.1 15.7 20.1 0.396 5.12 3.02 1.70 5.13  8.70 50 AA 1.2 51.0 22.0 23.6 0.409 5.25 2.55 2.06 6.19 12.75 51 AB 1.4 49.6 13.1 29.3 0.459 8.49 2.93 2.90 6.08 17.62 52 AC 1.4 46.3 30.1 20.3 0.489 7.29 3.03 2.41 5.44 13.09 53 AD 1.4 50.3 21.6 22.1 0.401 7.94 4.21 1.89 4.45  8.39 54 AE 1.2 54.1 17.6 22.8 0.378 6.15 2.08 2.96 5.34 15.79 55 AF 1.2 54.0 17.4 22.8 0.583 5.73 1.93 2.97 6.03 17.90 56 AG 1.4 52.2 21.4 21.3 0.422 4.96 3.04 1.63 5.90  9.63 57 AH 1.0 49.2 32.9 13.7 0.582 5.08 2.95 1.72 4.07  7.01 58 AI 1.4 48.5 30.0 15.9 0.354 4.36 2.76 1.58 5.12  8.09 59 AJ 1.4 51.0 20.2 20.5 0.447 5.58 3.32 1.68 4.80  8.07 60 AK 1.4 49.5 20.4 20.2 0.328 6.41 3.27 1.96 5.40 10.59 61 AL 1.2 50.2 20.1 20.6 0.311 6.87 2.60 2.64 4.13 10.91 62 AM 1.6 53.2 20.0 24.3 0.385 5.41 3.06 1.77 5.54  9.79 63 AN 1.4 52.7 20.3 20.6 0.335 4.77 1.69 2.82 4.52 12.76 64 AO 1.2 51.9 20.4 20.9 0.564 8.03 4.51 1.78 5.02  8.94 Underlined part: Outside of the scope of the present invention F: Ferrite, TM: Tempered martensite, RA: Retained austenite

TABLE 5 Average Average Content of C content C content diffusive Average Average in RA/ in RA/ hydrogen C content C content Average Average Balance in steel in RA in F C content Mn content micro- TS EL λ R (mass No. (mass %) (mass %) in F in RA structure (MPa) (%) (%) (mm) R/t ppm) Remark 1 0.47 0.04 11.50  0.07 M, BF, P, θ  997 22.4 27 2.6 2.2 0.03 Invention example 2 0.39 0.04 9.75 0.06 M, BF, P, θ 1023 25.0 26 3.4 2.4 0.06 Invention example 3 0.40 0.05 8.35 0.06 M, BF, P, θ 1006 25.8 27 3.0 1.9 0.16 Invention example 4 0.44 0.12 3.67 0.07 M, BF, P, θ 1099 21.0 24 2.2 1.8 0.13 Invention example 5 0.43 0.06 7.17 0.07 M, BF, P, θ 1024 26.1 15 2.4 2.0 0.14 Comparative example 6 0.33 0.03 11.00  0.06 M, BF, P, θ 1055 14.2 32 3.0 2.1 0.03 Comparative example 7 0.34 0.09 3.78 0.05 M, BF, P, θ 1183 13.1 23 5.6 4.0 0.05 Comparative example 8 0.37 0.06 6.17 0.06 M, BF, P, θ 1175 14.4 11 2.2 1.4 0.08 Comparative example 9 0.36 0.06 6.29 0.06 M, BF, P, θ 1172 13.2 14 3.2 2.3 0.02 Comparative example 10 0.19 0.04 4.54 0.02 M, BF, P, θ 1021 23.8 26 5.0 4.2 0.04 Comparative example 11 0.48 0.03 14.62  0.08 M, BF, P, θ 1052 18.5 19 3.2 2.3 0.10 Comparative example 12 0.46 0.08 5.98 0.08 M, BF, P, θ  983 18.9 19 1.6 1.3 0.15 Comparative example 13 0.43 0.09 4.78 0.06 M, BF, P, θ 1030 23.7 29 1.6 1.1 0.12 Invention example 14 0.44 0.10 4.40 0.09 M, BF, P, θ 1050 22.5 28 2.8 2.0 0.23 Invention example 15 0.45 0.09 5.00 0.07 M, BF, P, θ 1016 27.3 30 2.6 1.6 0.11 Invention example 16 0.46 0.06 7.51 0.08 M, BF, P, θ 1131 15.3 11 2.6 1.4 0.20 Comparative example 17 0.38 0.09 4.22 0.06 M, BF, P, θ 1021 24.6 12 1.8 1.5 0.18 Comparative example 18 0.48 0.12 4.00 0.07 M, BF, P, θ  985 25.7 16 3.2 2.3 0.15 Comparative example 19 0.33 0.07 4.71 0.07 M, BF, P, θ  943 29.7 33 0.8 0.6 0.09 Comparative example 20 0.33 0.14 2.36 0.09 M, BF, P, θ 1155 12.6 19 4.0 3.3 0.12 Comparative example 21 0.35 0.11 3.18 0.06 M, BF, P, θ  902 25.6 35 1.0 1.0 0.22 Comparative example 22 0.27 0.12 2.25 0.08 M, BF, P, θ 1123 13.7 24 3.8 3.2 0.06 Comparative example 23 0.45 0.06 7.50 0.08 M, BF, P, θ 1004 24.5 14 4.5 3.2 0.58 Comparative example 24 0.41 0.05 8.20 0.07 M, BF, P, θ 1022 16.4 26 2.0 1.4 0.04 Comparative example 25 0.41 0.08 5.13 0.07 M, BF, P, θ 1039 21.2 16 4.0 3.3 0.51 Comparative example 26 0.30 0.07 4.01 0.06 M, BF, P, θ 1046 20.7 22 2.4 1.7 0.23 Invention example 27 0.37 0.04 9.25 0.08 M, BF, P, θ  982 22.5 30 3.2 2.3 0.24 Invention example 28 0.38 0.09 4.22 0.09 M, BF, P, θ 1160 22.2 24 1.2 0.8 0.20 Invention example 29 0.50 0.05 10.00  0.07 M, BF, P, θ 1078 23.7 24 2.6 2.1 0.24 Invention example 30 0.46 0.04 11.50  0.06 M, BF, P, θ 1000 20.9 31 0.6 0.5 0.23 Invention example 31 0.31 0.07 4.43 0.05 M, BF, P, θ 1035 24.1 23 2.8 2.0 0.09 Invention example 32 0.48 0.09 5.46 0.10 M, BF, P, θ  988 26.2 23 2.6 2.2 0.17 Invention example 33 0.38 0.08 4.75 0.05 M, BF, P, θ 1003 28.0 28 3.2 1.8 0.02 Invention example 34 0.47 0.05 9.40 0.08 M, BF, P, θ 1085 21.0 25 2.3 1.7 0.05 Invention example 35 0.46 0.11 4.11 0.06 M, BF, P, θ 1052 24.1 27 2.2 1.8 0.14 Invention example 36 0.53 0.04 13.25  0.09 M, BF, P, θ 1021 22.1 26 2.8 2.3 0.12 Invention example 37 0.41 0.07 5.86 0.05 M, BF, P, θ  995 21.6 25 3.0 2.1 0.29 Invention example 38 0.44 0.05 8.80 0.06 M, BF, P, θ 1024 22.9 25 2.2 1.8 0.12 Invention example 39 0.25 0.06 4.17 0.05 M, BF, P, θ 1064 21.4 23 2.6 1.9 0.18 Invention example 40 0.33 0.02 16.50  0.06 M, BF, P, θ  841 15.2 26 2.2 1.6 0.05 Comparative example 41 0.37 0.04 9.23 0.05 M, BF, P, θ  989 15.1 32 5.0 3.6 0.21 Comparative example 42 0.47 0.06 7.83 0.07 M, BF, P, θ  984 16.0 26 4.0 3.3 0.22 Comparative example 43 0.37 0.07 5.29 0.04 M, BF, P, θ 1002 14.8 14 3.5 2.9 0.10 Comparative example 44 0.42 0.06 7.00 0.11 M, BF, P, θ 1025 13.5 20 2.6 2.2 0.10 Comparative example 45 0.52 0.11 4.73 0.11 M, BF, P, θ 1023 24.5 13 2.4 1.5 0.24 Comparative example 46 0.44 0.05 8.80 0.07 M, BF, P, θ 1012 25.9 24 1.8 1.3 0.08 Invention example 47 0.49 0.05 9.90 0.10 M, BF, P, θ 1129 23.2 28 2.4 1.7 0.19 Invention example 48 0.51 0.06 8.50 0.10 M, BF, P, θ 1000 22.9 25 2.6 1.6 0.23 Invention example 49 0.51 0.07 7.29 0.10 M, BF, P, θ  988 21.3 27 2.6 1.4 0.28 Invention example 50 0.54 0.07 7.71 0.10 M, BF, P, θ 1075 23.2 30 2.2 1.8 0.19 Invention example 51 0.44 0.09 4.89 0.05 M, BF, P, θ  983 30.4 25 2.6 1.9 0.09 Invention example 52 0.42 0.04 10.50  0.06 M, BF, P, θ 1082 20.3 18 2.6 1.9 0.03 Invention example 53 0.42 0.04 10.50  0.05 M, BF, P, θ 1014 21.0 25 1.2 0.9 0.22 Invention example 54 0.50 0.04 12.50  0.08 M, BF, P, θ 1002 21.4 28 2.6 2.2 0.15 Invention example 55 0.33 0.06 5.32 0.06 M, BF, P, θ  997 24.3 29 1.6 1.3 0.22 Invention example 56 0.45 0.09 5.00 0.09 M, BF, P, θ 1104 24.5 27 1.2 0.9 0.16 Invention example 57 0.34 0.10 3.50 0.07 M, BF, P, θ 1089 21.2 25 2.0 2.0 0.09 Invention example 58 0.41 0.06 6.83 0.09 M, BF, P, θ 1033 22.4 24 2.4 1.7 0.28 Invention example 59 0.37 0.12 3.08 0.07 M, BF, P, θ  997 26.7 27 2.8 2.0 0.11 Invention example 60 0.43 0.10 4.14 0.07 M, BF, P, θ  982 23.1 28 2.4 1.7 0.09 Invention example 61 0.46 0.07 6.57 0.07 M, BF, P, θ 1028 22.6 29 2.2 1.8 0.08 Invention example 62 0.45 0.09 5.00 0.08 M, BF, P, θ  995 26.8 27 2.8 1.8 0.06 Invention example 63 0.40 0.07 5.71 0.08 M, BF, P, θ  991 20.9 29 2.2 1.6 0.07 Invention example 64 0.44 0.03 14.67  0.06 M, BF, P, θ  986 22.1 28 2.8 2.3 0.08 Invention example Underlined part: Outside of the scope of the present invention F: Ferrite, M: Martensite, RA: Retained austenite, BF: Bainitic ferrite, P: Pearlite, θ: Carbides (e.g., cementite)

INDUSTRIAL APPLICABILITY

According to aspects of the present invention, a high strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability can be produced. Applying the high strength steel sheet according to aspects of the present invention to, for example, automotive structural components reduces the weights of car bodies and thereby improves fuel economy. Thus, the use of the high strength steel sheet according to aspects of the present invention is highly valuable from an industrial viewpoint. 

1. A high strength steel sheet comprising: a chemical composition containing, by mass, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: 2.50% or more and 8.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Al: 0.001% or more and 2.000% or less, with the balance being Fe and incidental impurities; a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein a quotient of an area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by a total area fraction of the retained austenite is 0.3 or more, wherein a quotient of an average Mn content (mass %) in the retained austenite divided by an average Mn content (mass %) in the ferrite is 1.5 or more, and a product of the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite and an average aspect ratio of the retained austenite is 3.0 or more, wherein a quotient of an average C content (mass %) in the retained austenite divided by an average C content (mass %) in the ferrite is 3.0 or more, wherein a quotient of the average C content (mass %) in the retained austenite divided by the average Mn content (mass %) in the retained austenite is 0.05 or more, and wherein a content of diffusible hydrogen in steel is 0.3 ppm by mass or less.
 2. The high strength steel sheet according to claim 1, wherein the chemical composition further contains at least one element selected from, by mass, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.0050% or less, and REM: 0.0050% or less.
 3. The high strength steel sheet according to claim 1, the high strength steel sheet further comprising a galvanized layer disposed on a surface of the high strength steel sheet.
 4. The high strength steel sheet according to claim 2, the high strength steel sheet further comprising a galvanized layer disposed on a surface of the high strength steel sheet.
 5. The high strength steel sheet according to claim 3, wherein the galvanized layer is a galvannealed layer.
 6. The high strength steel sheet according to claim 4, wherein the galvanized layer is a galvannealed layer.
 7. A method for manufacturing a high strength steel sheet according to claim 1, the method comprising heating a steel slab having the chemical composition, hot rolling the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then performing coiling at 300° C. or more and 750° C. or less, subsequently performing cold rolling, then performing holding at a temperature equal to or higher than an Ac₃ transformation temperature−50° C. for 20 s or more and 1800 s or less, subsequently performing cooling to a cooling stop temperature equal to or lower than a martensite transformation start temperature, then performing reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently performing holding at the reheating temperature for 2 s or more and 600 s or less, then performing cooling to room temperature, subsequently performing holding at a temperature equal to or higher than an Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, then performing cooling to 100° C. or less, then performing holding at higher than 100° C. and 400° C. or less for 10 s or more, and subsequently performing cooling.
 8. A method for manufacturing a high strength steel sheet according to claim 2, the method comprising heating a steel slab having the chemical composition, hot rolling the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then performing coiling at 300° C. or more and 750° C. or less, subsequently performing cold rolling, then performing holding at a temperature equal to or higher than an Ac₃ transformation temperature −50° C. for 20 s or more and 1800 s or less, subsequently performing cooling to a cooling stop temperature equal to or lower than a martensite transformation start temperature, then performing reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently performing holding at the reheating temperature for 2 s or more and 600 s or less, then performing cooling to room temperature, subsequently performing holding at a temperature equal to or higher than an Ac₁ transformation temperature and equal to or lower than the Aci transformation temperature+150° C. for 20 s or more and 600 s or less, then performing cooling to 100° C. or less, then performing holding at higher than 100° C. and 400° C. or less for 10 s or more, and subsequently performing cooling.
 9. The method for manufacturing a high strength steel sheet according to claim 7, the method further comprising, subsequent to the coiling and prior to the cold rolling, performing holding at a temperature equal to or lower than the Ac₁ transformation temperature for more than 1800 s.
 10. The method for manufacturing a high strength steel sheet according to claim 8, the method further comprising, subsequent to the coiling and prior to the cold rolling, performing holding at a temperature equal to or lower than the Ac₁ transformation temperature for more than 1800 s.
 11. The method for manufacturing a high strength steel sheet according to claim 7, the method further comprising, subsequent to the holding at the temperature equal to or higher than the Aci transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.
 12. The method for manufacturing a high strength steel sheet according to claim 8, the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.
 13. The method for manufacturing a high strength steel sheet according to claim 9, the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.
 14. The method for manufacturing a high strength steel sheet according to claim 10, the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac₁ transformation temperature and equal to or lower than the Ac₁ transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.
 15. The method for manufacturing a high strength steel sheet according to claim 11, the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.
 16. The method for manufacturing a high strength steel sheet according to claim 12, the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.
 17. The method for manufacturing a high strength steel sheet according to claim 13, the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.
 18. The method for manufacturing a high strength steel sheet according to claim 14, the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less. 